Human neutrophil heterogeneity has been suggested on the basis of differences chemotactic responsiveness and on the basis of results in our laboratory showing differences in Fc receptor mediated rosetting of IgG coated red cells. Heterogeneity of functional responsiveness was examined using the fluorescent activated cell sorter to assess functional responsiveness of neutrophil populations stimulated with a chemoattractant normally produced by E. coli, fmet-leu-phe (fMLP). Heterogeneity of enutrophil binding of fMLP was shown. In the majority of neutrophils initial binding was to a high affinity receptor with subsequent appearance of a low affinity receptor that was readily displaced with additional fMLP. The cells with the two classes of receptors proved to be the ones that responded with depolarization of membrane potential and reduction of nitroblue tetrazolium dye in response to fMLP. The remainder cells (10-20%) only exhibited the high affinity receptor for fMLP and lacked evidence for activation by fmet-leu-phe. We developed an IgG monoclonal antibody (31D8) that binds strongly to neutrophils that are activated with fMLP was developed. The 31D8 antigen first appears at the myelocyte stage of maturation. We used 31D8 to study patients with chronic myelogenous leukemia (CML), a clonal disease. In 5 patients no 31D8 positive cells were detected. These 5 patients, who were in remission at time of initial study, subsequently progressed to accelerated phase or blast crisis. In CML absence of 31D8 antigen expression may be an early marker for progression of CML. Early detection would be useful since bone marrow transplantation appears to be most efficacius in CML when done early. We have also demonstrated that neonate cord blood neutrophils have a decreased number of 31D8 positive neutrophils.